The present invention relates to an improved method for predicting the microstructure in which a cast iron melt, having a composition with a carbon equivalent near the eutectic point of the iron-carbon phase diagram, will solidify. The invention also relates to an apparatus for carrying out the method.
WO86/01755 (incorporated by reference) discloses a method for producing compacted graphite cast iron by using thermal analysis. A sample is taken from bath of molten cast iron and this sample is permitted to solidify during 0.5 to 10 minutes. The temperature is recorded simultaneously by two temperature-responsive means, one of which is arranged in the centre of the sample and the other in the immediate vicinity of the vessel wall. So-called cooling curves representing temperature of the iron sample as a function of time are recorded for each of the two temperature-responsive means. According to this document, it is then possible to determine the necessary amount of structure-modifying agents that must be added to the melt in order to obtain the desired microstructure. However, the cooling curves disclosed in this document are rather uniform and no variations are disclosed.
In order to accurately determine the graphite microstructure in cast iron specimens, conventional thermal analysis techniques, such as the one disclosed in WO 86/01755, require cooling curves where the first thermal arrest caused by austenite formation is distinctly separated from heat release caused by the onset of eutectic solidification. However, sometimes cooling curves are obtained without such a distinctly separated thermal arrest. This is the case when the molten cast iron solidifies as eutectic or hyper-eutectic iron. Until now, it has not been possible to use cooling curves corresponding to near eutectic cast iron for monitoring graphite growth.
WO93/20965 teaches that hyper-eutectic melts, where graphite nucleates before iron, do not provide a distinct plateau as the temperature crosses the liquidus line. This is correctly attributed to the fact that graphite crystallisation has a lower latent heat release than iron.
By placing a small amount of low-carbon iron in the melt, the low-carbon iron partially dissolves locally while the sample is still molten. As the sample cools, the relatively pure iron surrounding the remaining solid portion of the nail begins to solidify because of its lower carbon equivalent (CE). Ultimately, as the sample volume cools below the liquidus line, the remaining solid volume of nail, the surrounding low CE and the melted portion of the nail begin to solidify and xe2x80x9ctriggerxe2x80x9d the solidification in an otherwise hyper-eutectic melt. The net result is that an austenite arrest plateau appears on the cooling curve.
WO93/20965 also states that the temperature difference (xcex94T) between T67 and Tc max can be used to establish a correlation with the carbon equivalent. However, WO93/20965 refers to hyper-eutectic melts, i.e. melts where the carbon equivalent is so high that the release of heat from the primary solidification does not coincide with the minima of the cooling curve (in the hatched region of FIG. 2(a) of WO93/20965). Accordingly, the primary solidification and the eutectic solidification are separate.
None of the above cited references discuss anything about carrying out thermal analysis on cast iron melts in order to determine the carbon equivalent of melts which are near-eutectic. Moreover, WO93/20965 suggests measurements on melts having a carbon equivalent of up to 4.7%. It is disadvantageous to reach such high values because of graphite flotation and the degeneration of graphite shape. Likewise, the method of WO93/20965 is disadvantageous in that it requires an extra addition of low-carbon steel or iron to the sampling vessel, and accordingly lead to higher costs and a more laborious method.
The present invention provides a possibility to evaluate cooling curves recorded in near-eutectic cast iron melts. The curves are evaluated by determining the net amount of heat generated in the centre of the melt sample as a function of time. This information is then used to identify the part of the centrally recorded cooling curve that can be used as a basis for determining the amount of structure-modifying agent that must be added to produce compacted graphite cast iron, and/or spheroidal graphite cast iron, and to identify the part of said curve that is associated with formation of primary austenite.
The term xe2x80x9ccooling curvexe2x80x9d as utilised herein refers to graphs representing temperature as a function of time, which graphs have been recorded in the manner disclosed in WO86/01755, WO92/06809.
The term xe2x80x9cheat generation curvexe2x80x9d as utilised herein relates to a graph showing the heat that is generated in a certain zone of a molten cast iron. For the purposes of the present invention, all heat generation curves herein are determined for a zone located in the centre of a sample of molten cast iron. This zone is generally referred to as the xe2x80x9cA zonexe2x80x9d.
The term xe2x80x9csample vesselxe2x80x9d as disclosed herein, refers to a small sample container which, when used for thermal analysis, is filled with a sample of molten metal. The temperature is then recorded during solidification in a suitable way preferably the sample vessel is designed in the manner disclosed in WO86/01755, WO92/06809, WO91/13176 (incorporated by reference) and WO96/23206 (incorporated by reference).
The term xe2x80x9csampling devicexe2x80x9d as disclosed herein, refers to a device comprising a sample vessel equipped with at least two temperature responsive means for thermal analysis, said means being intended to be immersed in the solidifying metal sample during analysis, and a means for filling the sample with molten metal. The sample vessel is preferably equipped with said sensors in the manner disclosed in WO96/23206.
The term xe2x80x9cstructure-modifying agentxe2x80x9d as disclosed herein, relates to compounds affecting the morphology of graphite present in the molten cast iron. Suitable compounds can be chosen from the group of magnesium and rare earth metals such as cerium, or mixtures of these compounds. The relationship between the concentration of structure-modifying agents in molten cast irons and the graphite morphology of solidified cast irons have already been discussed in the above cited documents WO92/06809 and WO86/01755.
The term xe2x80x9cCGIxe2x80x9d as utilised herein refers to compacted graphite cast iron.
The term xe2x80x9cSGIxe2x80x9d as utilised herein refers to spheroidal graphite cast iron.